Horn antennas for both receiving or transmitting microwave energy are well known, and it is well known to make such horns of an electrically conductive material or of a non-conductive material that is plated or coated in some way with an electrically conductive material. Moreover, horn antennas that are specifically designed as receiving horns, of no matter what frequency, are required to be dimensioned and flared so as to achieve a concentration of low energy but discernable fields at one or more specific frequencies in the throat area of the horn. Detectors--often by way of a mixing diode or diodes--are inserted or placed at the throat of the horn so as to receive the energy that is present by way of those fields at the frequency or frequencies for which the horn has been designed.
Most often, in the case of receiving horns that are particularly for use and operation at microwave frequencies such as the X-band or K-band frequencies that are used for police speed detection radar and intruder detection radar for building or zone security, a highly sensitive horn is required. This is particularly so where the source of the radar or microwave frequency energy to be detected is at some considerable range from the detector; and in such instances, the physical placement of the mixing diode with respect to the throat of the horn is important. Generally, while planar aberrations can be corrected by a lens placed in front of the horn, there is still the requirement that there must be a positive coupling at the correct frequency between the field collector--the ridge--of the horn and the detecting circuitry such as the mixing diode or diodes. This usually requires that there be at least a tuning post positioned in the throat so as to be sure that the horn and its coupling operates at the correct frequency or frequencies, and this is at least in part brought about because of the physical placement of the mixing diode within the throat of the horn. These problems, therefore, carry with them the additional problem of frequency stability with temperature, and/or sensitivity, by virtue of the physical mounting and placement of the detecting circuitry with respect the throat and the field collector or ridge of the horn.
The present invention overcomes these problems by providing a positive, physically dimensionally stable coupling of the ridge of the horn to a feed strip for a mixing diode or pair of diodes mounted in association with a microstrip board; and the present invention also provides a horn antenna construction that is easy to mount using standard assembly line technology without the requirement for tuning or balancing the energy collection or detection at the throat of the horn.
In particular, the present invention overcomes those and other problems as discussed hereafter, by the provision of a horn antenna construction where a substantially rigid and planar mounting board having upper and lower surfaces has a microstrip board mounted on the upper surface thereof, where the microstrip board has a mixing diode or diodes mounted in association therewith, and a feed strip for the diode or diodes, and having the feed strip located at or near a first edge of the microstrip board on the upper, exposed side thereof. A one-piece molded horn element is provided that has an open bottom, an upwardly flared top wall and a pair of side walls, and having a downwardly extending ridge formed on the underside of the top wall. The ridge extends rearwardly to the throat of the horn, and extends downwardly at least in the throat area to such an extent that when the molded horn element is secured to the upper side of the mounting board, the bottom edge of the ridge in the throat area is brought into physical and electrical contact with the feed strip; and moreover, the upper surface of the mounting board then forms the bottom wall of the horn.
Provisions are also made according to the present invention for extremely good isolation among the high frequency input port from the horn, the local oscillator input frequency port from the local oscillator, and the intermediate (I.F.) output port, of the mixer which is mounted on the microstrip board. This is accomplished particularly by providing a pair of mixing diodes connected in anti-parallel, and one-quarter wavelength stub(s) for the operating frequency or frequencies of the horn on the far side of the mixing diodes from the feed strip, so as to provide a virtual short circuit at the far side of the mixing diode for the operating frequency or frequencies; and by dimensioning the horn throat so that it appears as a reactive impedance and thereby precludes outward radiation of energy at the local oscillator (L.O.) frequency but permits ground return of the L.O. signal; and by the provision of a low pass filter below the L.O. frequency so as to permit only I.F. frequency signals to exit from the I.F. port of the mixer.
The prior art includes ANDERSON ET AL, U.S. Pat. No. 3,339,275, issued Sept. 5, 1967. That patent teaches a method of making a horn antenna where two identical half sections are molded and secured together by flanges. The patent teaches the mounting of a centre conductor and a cone in the throat of the assembled horn. However, it must also be noted that the horn is intended for use over frequency ranges of 50 to 500 mHz; and has an overall length of eighteen feet and an aperture diameter of twelve feet.
PALMER, U.S. Pat. No. 3,530,482, issued Sept. 22, 1970, teaches a partitioned horn having variable flare sidewalls. Means are provided to change the physical dimensions, and thus the energy sensitivity, in the throat of the horn.
RISKO, U.S. Pat. No. 4,058,813, issued Nov. 15, 1977, teaches a wave guide horn antenna that may be assembled from sheet metal, and which may have several different embodiments. In any event, the horn antenna that is taught in that patent is one which still requires a tuning screw in the throat of the horn, and is one where the precise physical dimensions of the horn and thus its energy frequency sensitivity are not predictable due to the manufacturing processes that are employed.